The objective of the proposed research is to test the hypothesis that viscous forces play a non-trivial role in transport processes within small diameter, porous tubes, specifically within renal tubules. The hypothesis will be tested by investigating, quantitatively, the effects of viscous forces in a number of model systems applicable to physiological situations. Specifically, an analytical solution of the Navier-Stokes equations can be obtained if certain assumptions and approximations, consistent with biological data, are made. The analytical solution will be used, in conjunction with model systems, to study (1) the possibility that viscous forces contribute to the phenomenon of glomerulo-tubular balance in renal proximal tubules; (2) the possibility that pressure and viscosity play significant roles in initiating renal medullary counter-current multiplication, as well as counter-current multiplication in other physiological systems.